The gears are the 3 speed AW hub with the 54 tooth chain ring (+8%). I guess this could be too high, but it makes the middle gear more usable, I do try to not use the high gear all the time and make sure that I am spinning (as I would on my mtb or road bike). This was one of the thoughs about a new bike, the chance to get more gears, may help with getting a better gear. With only three gears, they are quite well spaced apart, often neither the high or lower fits your terrain/wind speed and you seek an inbetween gear. Would a 6 speed Brompton give me the fill in gears or just more higher and lowerr than my current range?

The bars are the old original style M type, which makes the riding position quite upright, hence my thoughts about an S-type.

On makingmarks suggestions, I am not offended and is a good suggestion however I think this is a genuine problem, I have had a history of knee problems, in the past brought on by running or hiking with heavy loads. I am content in work and home life so I think that I can rule this out.